Occasionally, when I stand up after laying flat on a bed or couch I will take a few steps and then start to have tunnel/ darkened vision, a slight headache, and sometimes lose some balance, once falling down and only keeping from hitting the ground by instinct, and then I forgot how I had left my bed for a few seconds.
While I am not to worried about this, I would like to know what might be causing this, so I can try to avoid a more serious accident. Could dehydration cause this? I have some friends who report similar experiences but others deny having them. Are some people more inclined to dizziness from this? I don't think it is from exertion or anything, as I am resonably get and recently completed a weeklong backpacking trip, although it is not something I usually do.